ECMS-6234 PropertyValueComparator class is duplicated in both core service and webui

Fix description:
- Delete PropertyValueComparator class in webui.
- Only use PropertyValueComparator in core service.
- Update compare() method of the kept PropertyValueComparator to handle the case of missing properties
  such as owner, publication state of a folder, or author of built-in folders.

Conflicts:
	core/webui-explorer/src/main/java/org/exoplatform/ecm/webui/component/explorer/UIJCRExplorer.java
ECMS-6234 PropertyValueComparator class is duplicated in core service and in webui
Fix description: re-add PropertyValueComparator in core webui to use in platform.